text: Archaerhodopsins drive the hyperpolarization of the cell membrane by secreting protons in presence of light, thereby inhibiting action potential firing of neurons. This process is associated with an increase in extracellular H+ (i.e. decreased pH linked to the activity of these proteins. These characteristics allow for Archaerhodopsins to be commonly used tools for optogenetic studies as they behave as transmission inhibition factors in presence of light. When expressed within intracellular membranes, the proton pump activity increases the cytosolic pH, this functionality can be used for optogenetic acidification of lysosomes and synaptic vesicles when targeted to these organelles.